Make QML ListView work with QScroller

The main complication in ListView's use of Flickable is that it must support items with unknown size. This means that ListView does not know the exact size of the contents. It estimates the boundaries as it is flicked and adjusts the animation accordingly. This applies to both the beginning and end of the view, and the boundaries of individual delegates when a snapping mode is enabled. Note that the beginning is also not necessarily 0.